Free cutting alloy



Patented Apr. 13, 1937 UNITED STATES FREE CUTTING ALLOY Louis W. Kempfand Walter A. Dean, Cleveland, Ohio, assignors to Aluminum Company ofAmerica, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.Original application December 28,

1935, Serial No. 56,552. Divided and this application July 27, 1936,Serial No. 92,925

, 3 Claims.

The invention relates to aluminum base alloys and is particularlyconcerned with aluminum base alloys containing magnesium and silicon.

This application is a division of our co-pending application, Serial No.56,552, filed December 28, 1935. Alloys disclosed butnot claimed hereinare claimed in the aforesaid co-pending application, Serial No. 56,552.

Aluminum base alloys containing from about 0.1 per cent to'1.0 per centof magnesium, and from about 0.5 per cent to about 2.5 per cent ofsilicon have heretofore been used. They are usually in the wroughtcondition since they are readily susceptible to mechanical deformationsuch as rolling, forging, or extrusion. It is possible, by appropriatethermal treatment, to very substantially improve their physicalproperties. After a high temperature solution treatment they do notage-harden to any great extent at room temperature but may beartificially aged by temperatures somewhat in excess of room temperatureas is well known in the art. For this reason these alloys may be wroughtto substantially final form, then subjected to a high temperaturesolution treatment, and while in a relatively soft condition they maybegiven certain final working steps at low.temperature. Since they donot materially age at room temperature these finishing steps may beperformed at any convenient time. They may ultimately be artificiallyaged if the maximum tensile strength, yield strength, and hardness aredesired.

Since all commercial aluminum base alloys contain some silicon as animpurity, usually less than, or not greatly in excess of 0.5 per cent,it is to be understood that the amount of silicon discussed herein, andhereinafter claimed, representsthe total quantity of silicon present,and

that the-silicon content of the aluminum used should loci-known in orderto provide a basis for determining the eventual composition of thealuminum-magnesium-silicon alloy.

There are, however, some applications wherein aluminum-magnesium-siliconalloys as hereinabove disclosed might be conveniently and profitablyused except for an inherent disadvantage which militates against theiruse in the production of certain articles requiring exacting machiningoperations. Mechanical cutting operations such as drilling, shaping, orlathe-cutting are successfully carried out only by using certainprecautions which increase the cost of production and perhaps favor thechoice of another metal or alloy which can be machined more readily butwhich is not so desirable in other respects, as for example, in physicalproperties. When alloys are difficult to machine this disadvantagebecomes evident, in many cases through rapid wear of the cutting tooledge, so that frequent tool re-sharpening is required. Despite continuallubrication the machined surface is rough and irregular, and the chiphas atendency to form a continuous curl or spiral which often fouls thetool or the moving parts of the machine. It is immediately apparent thatthere is need for an alloy of good working characteristics andsatisfactory physical properties, yet possessing such favorablemachining properties that the complete machining operations may beperformed economically and successfully, and may be productive of apleasing surface appearance.

Accordingly an object of our invention is the provision of an aluminumbase alloy containing from about 0.1 to about 1.5 per cent of magnesiumand from about 0.5 to 2.5 per cent of silicon, which may be readily andeconomically machined.

Our invention resides in the discovery that the foregoing object iseffected by adding at least one of the elements lead and tin to thealuminummagnesium-silicon alloy. When these elements are added, in theproportions specified below, the alloys are known as free cutting orfree machining alloys because they can be -machined more rapidly thansimilar alloys without the lead or tin, and yet have as good or a betterfinished surface. After an extended series of investigations we havediscovered that theelements lead and tin belong to a class of alloyingelements and tin free machining elements. We have further discoveredthat the simultaneous presence of both of these elements is productiveof an improvement infree machining characteristics which is considerablygreater than that caused by the presence of the same total amount ofeither one alone. For example, the addition of 0.5 per cent of lead and0.5 per cent of tin to an aluminum base alloy containing about 0.6 percent of magnesium and 1.0 per cent of silicon effects a great-' erimprovement in machining quality than does the addition of 1.0 per centof either lead or tin singly.

Since these two elements have such a favorable effect on the machiningcharacteristics of aluminum-magnesium-silicon alloys, it is fortunatethat they are also of relatively low melting point, a fact which makespossible their addition to molten aluminum in the pure state, withoutthe intervention of so-called rich alloys. It is also a fact thattheelements lead and tin form with aluminum a series of alloys oflimited liquid solubility. Within the range disclosed and claimed,however, these free machining elements may be added without unusualdifllculty. We suspect that this characteristic feature of the elementslead and tin may be one of the significant factors which contribute totheir free machining efiect. We believe that this effect is furtherstrengthened by distributing the free machining constituent relativelyhomogeneously throughout thesolid matrix, since lead and tin are alsopractically insoluble in the solid aluminum-magnesium-silicon basealloy.

The lead or tin, if used alone, may be added in amounts between about0.05 per cent and 6.0 per cent. If used together the total amount of 29free machining constituent should not be less than 0.05 per cent, andneed not exceed 6.0 per cent. We have determined that the maximum limitof 6.0 per cent of free machining metal is suflicient for satisfactorycommercial results, since alv 25 though the free machining eiiectpersists beyond this amount, certain of the other physical propertie-umay be unfavorably ailected.

Aluminum-magnesium-silicon alloys containing one or 'both of the freemachining elements lead or tin may be machined more rapidly, with lesstool wear, less tool sharpening, better quality of chip and bettermachined surface than the ,same base alloys without the free machiningadditions and as previously indicated the simultaneous presence of tinand lead is considerably more helpful to the machining quality of thealloy than an equivalent total amount of either lead or tin alone.

As a preferred alloy within the range of the 40 respective elementsdisclosed hereinabove we su gest an alloy containing about 0.6 per centof magnesium, 1.0 per cent of silicon and a total of 3 per cent of leadand tin, the balance being aluminum. For some applications which do not5 require a high degree, of free machining quality a lesser amount ofthe free machining metal may be added to the alloy base, for instance0.5 per cent of lead or 0.5 per cent of tin, or 0.75 per cent total oflead and tin. .For certain purposes, notably the improvement of tensilestrength, hardness and grain structure, the alloys as hereinabovedisclosed may be improved by the addition of one or more of the group ofelements composed of molybdenum, vanadium, 55 titanium, tungsten,zirconium and chromium.

, From 0.05 to 1 per cent of any one of these elev ments may be usedalone, but if more than one is employed the total amount should notexceed about 2 per cent.

mal'treatments well known in the art to improve their strength andhardness. We have found that a solution heat treatment and subsequentaging does not impair the free machining quality of the alloys and inmany instances the treatment even tends to improve this property. Formanypurposes a relatively high strength and hardness are necessary tothe successful performance of the machined article and hence the alloymust be heat treated. This treatment is generally applied prior to themachining operation.

As hereinabove indicated the free machining elements, by reason of theirlow melting point, may be added to the molten aluminum in pure metallicform. However, since some difflculty may be encountered in introducingthem in the higher percentages of our disclosed range we prefer to usethe method which is more fully described in U. S; Patent No. 1,959,029,issued March 15, 1934. Briefly it involves heating the melt to asomewhat higher temperature than is customary, and vigorously stirringit in excess of a critical period of time.

'The term aluminum as used herein and in the appended claims embracesthe usual impurities found in aluminum ingot of commercial grade .orpicked up-in the course of the ordinary handling operations incident tomelting practice.

We claim: 1. A free cutting alloy containing from 0.1 to 1.5 per centmagnesium, from 0.5 to 2.5 per cent silicon, from 0.05 to 6.0 per centlead and from 0.05 to 6 per cent tin, the total amount of lead and tinbeing not in excess of about 6.0 per cent, the balance beingsubstantially aluminum.

2. A free cutting alloy containing from 0.1 to 1.5 per cent magnesium,from 0.5 to 2.5 per cent silicon, from 0.05 to 6.0 per cent lead andfrom 0.05 to 6.0 per cent tin, the total amount of lead and tin beingnot in excess of about 6.0 per cent, and from 0.05 to 2 per cent ofhardening metal from the group composed of molybdenum, vanadium,titanium, tungsten, zirconium, and chromium, the balance beingsubstantially aluminum.

3. A free cutting alloy containing about 0.6 per cent magnesium, 1.0 percent silicon, 0.5 per cent lead and 0.5 per cent tin, the balance beingaluminum.

LOUIS W. KEMPF. WAL'I'ER. A. DEAN.

